Microporous filaments and fibers, and articles made therefrom

Abstract

Fibers and filaments of an ultra-high-molecular-weight polyolefin that have a void volume greater than about 20% and are wettble. Wettability is imparted to the fibers and filaments by incorporating a filler into the composition that is hygroscopic, has a particle size less than about 10 microns in diameter and a surface area greater than about 30 square meters per gram. The preferred fillers are those that contain surface silanol groups. The fibers and filaments are formed by the process of preparing a mixture of an ultra-high-molecular-weight polyolefin, filler and plasticizer, metering the mixture to an extruder, heating and kneading the blend in the extruder, conveying the extrudate to a fiber or filament forming type die, expressing the extrudate through the die openings to form fibers or filaments, and extracting at least a portion of the extractable plasticizer to provide the desired porosity. The fibers and filaments may be formed into nonwoven webs directly by the melt blown or spun bonded process or by airlaying or wetlaying techniques. Such webs are particularly useful as a battery separator. The filaments may also be woven into fabric.

We claim: 
     
       1. A battery separator comprised of a web formed of a multiplicity of microporous fibers or filaments, the composition of said microporous fibers or filaments being comprised of between about 4% to about 90% by volume of an ultrahigh-molecular weight polyolefin selected from the group consisting of homopolymers and copolymers of ethylene, propylene and butylene; between about 10% to about 96% by volume of a hygroscopic filler having a particle size less than about 10 microns; said microporous fibers or filaments having a void volume greater than about 20%;   said web having: a thickness of between about 1 and about 120 mils; a macroporosity formed by the interstices between said microporous fibers or filaments; and an electrical resistance of less than about 1.5 milliohms per square inch per mil thickness.   
     
     
       2. The battery separator of claim 1 having a thickness between about 1 and about 15 mils. 
     
     
       3. The battery separator of claim 1 having a thickness between about 15 and about 120 mils. 
     
     
       4. The battery separator of any of claims 1, 2 or 3 wherein the ratio of filler to polyolefin is between about 1:1 and about 20:1. 
     
     
       5. The battery separator of claim 1 wherein the filler comprises between about 60% and about 96% by volume. 
     
     
       6. The battery separator of any of claims 1, 4 or 5 wherein the filler is silica. 
     
     
       7. The battery separator of any of claim 1, 4, 5 or 6 wherein the polyolefin is ultra-high-molecular-weight polyethylene. 
     
     
       8. A battery separator for use in a recombinant type lead acid battery comprising a web formed of a multiplicity of microporous fibers or filaments, the composition of said microporous fibers or filaments being between about 4% to about 90% by volume of an ultra-high-molecular weight polyethylene; between about 10% to about 96% by volume of silica having a particle size less than about 10 microns;   said web having: a thickness of between about 15 and about 120 mils; a macroporosity formed by the interstices between said microporous fibers or filaments; and an electrical resistance of less than about 1.5 milliohms per square inch per mil thickness.   
     
     
       9. The battery separator of claim 8 wherein the silica content of the microporous fiber or filament is between about 60% and about 96% by volume. 
     
     
       10. The battery separator of claim 8 or claim 9 wherein the average diameter of the microporous fibers or filaments is less than about 10 microns.